1. Field of the Invention
The present invention relates to resonant antennas. More particularly, the present invention relates to tunable antennas and to a portable tunable Yagi antenna.
2. The Prior Art
The initial assembly of current portable Yagi antennas, as well as most vertical and dipole antennas, takes a considerable amount of time. Worse yet, to change frequency and adjust the antenna for a good feedline match requires taking the antenna down and making mechanical and or electrical adjustments. Despite all of the effort required, currently-available portable Yagi antennas offer a very low level of performance.
Existing designs use one or more of the following techniques to change frequency: physically adding lengths of aluminum to the element(s), introducing inductance to change the electrical element length, either by swapping fixed inductors in and out to effect band changes or by providing a motor driven variable inductor. In the case of manually swapping the coils it is usually also necessary to also change the physical length of the antenna. These procedures are very time consuming and results in a very lossy antenna. The motor driven inductor types of antennas suffer additional losses because the maximum physical length is determined by the highest frequency at which the antenna operates, thus reducing efficiency greatly on all frequencies below the upper limit because the fixed radiator is much too short. Because of the large values of inductance required, current portable antennas have relatively modest power handling ability, typically about 100 w to 500 w. To solve these problems a new type of element is needed.